Electrolytic production of laminated metal articles



April 1960 w. w. GULLETT ET AL 2,933,439

ELECTROLYTIC PRODUCTION OF LAMINATED METAL ARTICLES Filed Feb. 25, 1957 ELECTROLYTIQ PRQDUCTFQN F LAMINATED METAL ARTICLES William W. Gullett, College Park, and Frank X. McCawley, Cheverly, Md, assignorsto Chicago Development Corporation, Riverdale, Md a corporation of Delaware Application February 25, 1957, fierial No. 642,677

' 2 Claims. c1. 204-47 This invention relates to high melting metals in the form of continuous nonporous ductile layers over metal surfaces, and particularly to such layers formed by the consolidation of rhythmic precipitation bands of crystals of such metals as titanium, zirconium, molybdenum, chromium and vanadium produced by reaction of alkalinous metal solutions and higher melting metal .chloride solutions in molten alkalinous metal chlorides.

By the term alkalinous metals" is meant the alkali and'alkaline earth metals.

linous metal.

A further object is to produce such layers in highly ductile form and with a minimum of inter-diffusion of the high-melting metal of theouter layer and the metal of the cathode.

" We have found that when a direct current is passed between an inert cathode and an anode of a metal selected from the group consisting of titanium, zirconium, molybdenum, chromium and vanadium in a molten elecitrolyte consisting essentially of alkalinous metal chloride and dissolved therein chlorides of the metal ofthe layer to be formed and free alltalinous metal, the current being such that the instantaneous open circuit voltage of the cell is 2-30 millivolts, there are formed around the cathode rhythmic precipitation bands. Such bands are well known where a precipitation reaction proceeds under conditions in which supersaturation takes place ahead of precipitation: see Ostwald, Lehrbuch Allgemeine Chemie, 2nd ed., vol. 2, page 778.

The existence of super-saturated compositions in electrolytes of the type described are also known.

In the process of the present invention no electrodeposition of the high melting metal takes place because the cathode potential is only high enough to form a solution of alkalinous metal, not to discharge any higher melting metal ion present. The solution of alkalinous ,metal supersaturates the bath and a seriescfrhythmic bands of higher melting metal crystals are formed about the cathode.

This is the first step of the present invention, and it may be defined by both composition and cathode potential; or, less accurately, by cathodecurrent density.

Withthese parameters defined within the limits of our invention, a series of rhythmic bands extending to 25 or 30 mils are formed.

This step doesnot, in and of itself, provide a continuous and non-porous layer over the cathode metal surface,

but rather the condition shown in the photomicrograph Fig. 1. This photomicrograph, at 300 diameters, shows the rhythmic bands formed in 20 minutes at 2 amperes per sq. ft. at 800? C., in a bath consisting essentially of NaCl and dissolved therein 7% titanium chloride, average valence 2.5 and 0.5% metallic sodium, the analyses being made by the method described in Contributions to Titanium Metallurgy, Papers #1, 2 and 3, by Chicago Development Corporation.

In connection with Fig. 1 the following are noted:

This photomicrograph, at 300x magnificatiomis of a cross-section of a portion of a low-carbon steel cathode with material (including salt) attached to its surface. Because the salt could not be etched, the sample was polished. with dry emery and hence the steel was scratched. In this view, the cathode is shown'at l. Adhering to the cathode surface is a thin band 2 of salt to which latter there adheres a first rhythmic band 3 of loosely consolidated titanium crystals. Beyond band 3 is a larger layer 4 of salt in which there had-during the operation-begun to form a second rhythmic band of titanium crystals as appears at 5, 5.

The second step in the process of. our invention is the consolidation of the inner "precipitation bands into a single non-porous layer shown in Fig. 2 (to be referred to hereinafter). This result is accomplished by heating the deposit in the bath for anadditional period of several hours. During 'this period it is not necessary to pass current, but if the same is done the outer rhythmic bands so formed will remain unconsolidated and can be removed from the finished product by brushing.

If the cathode metal is a high melting metal such as molybdenum the consolidated outer layer will be ductile and only a minimum of inter-dillusion between the'cathode metal and the metal of the outer layer will-take place.

In the case of a cathode composed of ferrous metal (iron, steel or ferrous alloy), however, the consolidation step is sometimes accompanied by enough inter-dififusion so that the continuous and non-porous outer layer is brittle. Since this condition frequently is obiectionable, we may, and prefer to, provide over the cathode a barrier layer to prevent such diffusion. This barrier is conveniently a thin layer of carbide of the higher melting metal, which carbide may be produced by providing a graphite coating on the ferrous metal or other metal tending'to difiuse intothe metal o'f the outer layer at the temperature of consolidation. The resulting thin diffusion barrier of carbide of the high-melting metal of the outer layer does not'prevent the Working of the base metal and its coating layer without separation of layers, since the carbide layer breaks into small fragments and permits bonding of the layers on working particularly at elevated temperature.

The result of consolidation, and sintering, of the rhythmic bands of titanium on the cathode surface is pictured in Fig. 2, which is 'a photomicrograph, at 30X magnification, of an etched cross-section of a portion of a low-carbon steel cathode provided with a continuous non-porous adherent layer of titanium. The

same shows at 1 the lower-carbon steel cathode with characteristic inclusions 6 of perlite therein. At 7 appears the continuous non-porous adherent layer of titanium, the junction between steel and titanium appearing in line 8 therebetween.

The uses of the product of this invention will be clear to those skilled in the art. For example a titanium-clad metal'article will have the same exceptional corrosion resistance as that of an article formed of the pure metal, and will also possess the properties of the basis metal.

Having now described our process and product, we will illustrate it by examples.

Composition of bath:

In all these examples, we use a fused bath composed of alkalinous chloride, higher melting metal chloride in soluble form and dissolved alkalinous metal, held in a heated iron pot provided with a cover; a means for maintaining an argon atmosphere over said bath, a basket of higher melting metal chips as an anode .sur-

rounding the cathode; and a means for removing the cathode from the bath and for cooling the same in argon.

The cathode with its layer of metal is washed after cooling to remove salt and unconsolidated metal crystals.

Example 1 Composition of bath: NaCl base electrolyte, containing dissolved therein 7% soluble Ti, average valence 2.5,

and 0.2% sodium. Metal to be coated: molybdenum Current, per sq. ft. of metal to be coated: 25 amps.

Instantaneous open circuitvoltage of cell: 10 millivolts Current, per sq. ft. of comminuted anode material: 1

amp. l

Time of step 1: 3 hours Time of step 2: 5 hours Thickness of consolidated layer: 2.0 mils Temperature: 800 C.

The consolidated layer of titanium was continuous, adherent, ductile and non-porous. It was not spalled or cracked by severe bending of the clad article.

Example 2 Composition of bath: NaCl, base electrolyte, containing dissolved therein 2% sol. Ti, average valence 2.05, and sodium .02%.

Metal to be coated: Iron, coated with graphite Current per sq. ft. of metal to be coated: 32 amps.

Instantaneous open circuit voltage of cell: 12 m.v.

Current per sq. ft. of Ti anode material: less than 1 amp.

Time of step 1: hours Time of step 2: 6 hours Thickness of consolidated layer: 6 mils Temperature: 800 C.

Consolidated Ti layer was continuous, adherent, non-, porous, and ductile.

It could be reduced 50% in thickness by cold rolling without spalling. Alternate immersion in saltwater showed the clad iron to be completely protected from corrosion.

. Example 3 Composition of bath:

Base electrolyte, containing dissolved therein 65% SrCl 35% NaCl Soluble Ti 8%, average valence 2.5, and alkali metal 7 Metal to be coated: uranium, coated with graphite Current per sq. ft. of metal to be coated: 10 amperes Inst. O.C. voltage of cell: 4 m.v. Current per sq. ft. of Ti anode material: less than 1 amp.

Time of step 1: 8 hours Time of step 2: 8 hours Thickness of consolidated layer: 2 mils Temperature: 600 C.

The Ti coating was continuous, adherent, ductile and non-porous. Theuranium was reduced 5 0% in thickness without cracking the consolidated layer, which latter remained fully protective against corrosion.

Example 4 NaCl base electrolyte, containing dissolved therein soluble zirconium 6%, average I valence'2.4, and sodium 4%.

Metal to be coated: graphite-coated nickel Current per sq. ft. of metal to be coated: 50 amps.

Inst. O.C. voltage of cell: 14-m.v.'

Current per sq. ft.;of Zr anode material: less than 1 Time of step 1: 3 hours Time of step 2: 8 hours Thickness of consolidated layer: 4 mils Temperature: 850 C.

The'consolidated zirconium layer was continuous, adherent, ductile and non-porous.

Example 5 The consolidated molybdenum layer was continuous, adherent, ductile and non-porous.

Example 6 Composition of bath: NaCl base electrolyte, containing dissolved therein soluble Cr 5%, average valence 2.01, and sodium 0.3%.

Metal to be coated: graphite-coated steel Current per sq. ft. of metal to be coated: 30 amps.

Inst. O.C. voltage of cell: 8 m.v.

Current per sq. ft. of Cr anode material: less than 1 amp.

Time of step 1: 3 hours Time of step 2: 30 hours 7 Thickness of consolidated layer: 5 mils Temperature: 900C.

The consolidated chromium layer was continuous, ductile and non-porous.

Example 7 Composition of bath: BaCl base electrolyte, containing dissolved therein soluble V 10%, average valence 2.6 and alkalinous metal 1.8%

Plating conditions and nature of product were identical with'those of Example 6. The consolidated vanadium layer was continuous, ductile and non-porous.

Example 8 Composition of bath-as in Example 3.

Metal to be coated: magnesium Current per sq. ft. metal to be coated: 10 amperes Inst. O.C. voltage of cell: 20 m.v. Current per sq'. ft. of titanium anode: less than lamp. Time of step 1: 15 minutes Time of step 2: 10 minutes 7 Thickness of consolidated layer: 1 mil Temperature: 550 C.

The titanium coating was continuous, adherent, ductile, nonporous and fully protective.

Example 9 In this example we proceed exactly as in Example 8 ex-.

cept that the base metal is aluminum. Coating was similar in all respects as Example 8. t

Example 10 Composition of bath: as in Example 3. Metal to be coated: copper coated with graphite Current per sq. ft. of metal to be coated: 20 amperes" Inst. O.C. voltage of cell: 10 m.v. r Current per sq. ft. titanium anode: less than 1 amp. Timeofstep 1: 4hours i iii-f Time of step 2: simultaneous with step 1 Thickness of consolidated layer: mils. Temperature: 750 C.

The titanium coating was continuous, adherent, ductile and non-porous. No intermediate alloy layer was formed.

We claim:

1. The method of producing a continuous non-porous ductile outer layer of a polyvalent metal selected from the group consisting of Ti, Zr, Mo, Cr and V on but not alloyed with a basis metal selected from the group consisting of molybdenum, tungsten, an alloy consisting principally of molybdenum and an alloy consisting principally of tungsten, which comprises making the basis metal a cathode in an electrolytic cell having an anode of the metal of such outer layer metal, an inert atmosphere and an electrolyte of at least one alkalinous metal chloride containing in solution 2-10% of the metal of the outer layer as soluble chloride, average valence 2.05- 2.7, and 0.0l2.5% dissolved free alkalinous metal, passing a direct current between anode and cathode at a temperature above the melting point of the bath and up to 900 C., said current being such that the instantaneous open circuit voltage of the cell is 2-30 millivolts, the cathode potential being insufiicient to discharge ions of any metal present in the bath of higher melting point than the alkalinous metal, passing said current for a period of from 10 minutes to 6 hours whereby to form rhythmic precipitation bands of crystals of said outer layer metal about the cathode, heating said cathode with surrounding crystals for a total period of 10 minutes- 30 hours, at said elevated temperature, whereby to consolidate the crystals in the bands into a continuous nonporous and ductile layer firmly attached to the basis metal, removing the cathode and attached layer from the bath, cooling in a neutral atmosphere, and Washing ofi salt and unconsolidated outer layer metal crystals.

2. The method of claim 1, in which the cathode is composed of a metal having a melting point higher'than 1000" C. and is provided with a coating of graphite before being placed in the cell, whereby to provide be tween the cathode and the outer layer metal a thin barrier preventing interdiffusion of the outer layer of metal and the cathode.

I References Cited in the file of this patent UNITED STATES PATENTS 2,332,737 Marvin Oct. 26, 1943 2,683,671 Findlay July 13, 1954 2,715,093 Senderofi? et al. Aug. 9, 1955 2,731,403 Rubin Ian. 17, 1956 2,748,067 Pease May 29, 1956 2,828,251 Sibert et a1. Mar. 25, 1958 2,838,393 Dean June 10, 1958 FOREIGN PATENTS 788,804 Great Britain Jan. 8, 1958 

1. THE METHOD OF PRODUCING A CONTINUOUS NON-POROUS DUCTILE OUTER LAYER OF A POLYVALENT METAL SELECTED FROM THE GROUP CONSISTING OF TI, ZR, MO, CR AND V ON BUT NOT ALLOYED WITH A BASIS METAL SELECTED FROM THE GROUP CONSISTING MOLYBDENUM, TUNGSTEN, AN ALLOY CONSISTING PRINCIPALLY OF MOLYBDENUM AND AN ALLOY CONSISTING PRINCIPALLY OF TUNGSTEN, WHICH COMPRISES MAKING THE BASIS METAL A CATHODE IN AN ELECTROLYTIC CELL HAVING AN ANODE OF THE METAL OF SUCH OUTER LAYER METAL, AN INERT ATMOS PHERE AND AN ELECTROLYTE OF AT LEAST ONE ALKALINOUS METAL CHLORIDE CONTAINING IN SOLUTION 2-10% OF THE METAL OF THE OUTER LAYER AS SOLUBLE CHLORIDE, AVERAGE VALENCE 2.052.7, AND 0.01-2.5% DISSOLVED FREE ALKALINOUS METAL, PASSING A DIRECT CURRENT BETWEEN ANODE AND CATHODE AT A TEMPERATURE ABOVE THE MELTING POINT OF THE BATH AND UP TO 900*C., SAID CURRENT BEING SUCH THAT THE INSTANTANEOUS OPEN CIRCUIT VOLTAGE OF THE CELL IS 2-30 MILLIVOLTS, THE CATHODE POTENTIAL BEING INSUFFICIENT TO DISCHARGE IONS OF ANY METAL PRESENT IN THE BATH OF HIGHER MELTING POINT THAN THE ALKALINOUS METAL, PASSING SAID CURRENT FOR A PERIOD OF FROM 10 MINUTES TO 6 HOURS WHEREBY TO FROM RHYTHMIC PRECIPITATION BANDS OF CRYSTALS OF SAID OUTER LAYER METAL ABOUT THE CATHODE, HEATING SAID CATHODE WITH SURROUNDING CRYSTALS FOR A TOTAL PERIOD OF 10 MINUTES30 HOURS, AT SAID ELEVATED TEMPERATURE, WHEREBY TO CONSOLIDATE THE CRYSTALS IN THE BANDS INTO A CONTINUOUS NONPOROUS AND DUCTILE LAYER FIRMLY ATTACHED TO THE BASIS METAL, REMOVING THE CATHODE AND ATTACHED LAYER FROM THE BATH, COOLING IN A NEUTRAL ATMOSPHERE, AND WASHING OFF SALT AND UNCONSOLIDATED OUTER LAYER METAL CRYSTALS. 